1. Field of the Invention
This invention relates to a device for determining an operating parameter signal for a load fed by a voltage, such as for determining a flux signal for a rotary field machine.
2. Description of the Prior Art
To determine an operating parameter from a harmonic-affected or otherwise disturbed a-c voltage in a computing circuit (e.g. to form a load control signal from the feed voltage of a connected load in a control system), smoothers are often used which have undiminished transmittance for low frequency voltages, but which damp high frequencies. Such smoothers have a frequency response 1/(1+s t), with a time constant t and a laplacian operator s, and are generally designed as integrating circuits having resistors connected in parallel with the integrating capacitors. Besides damping the undesired harmonics, however, smoothers of this type bring about a phase shift of the a-c voltage. This shift must be accepted, since a transfer stage with the transfer function (1+s t) needed to compensate for it, would in turn amplify the damped harmonics again.
It is gnerally not possible to apply the load voltage on which the calculation of the parameter is based directly to the computing circuit. On the one hand, a d-c isolation of the computing circuit from the voltage network is required, and on the other hand, the operational amplifiers and other components used in the computing circuit operate at a relatively low voltage level which usually lies considerably below the load voltage level. For this reason, transformer circuitry is required between the voltage network and the computing circuit to provide d-c isolation and an appropriate response ratio.
For a rotary machine voltage-fed load, good operating control of the machine can be achieved using a magnetic flux signal formed by integrating the machine EMF. The EMF can be calculated from the machine voltage and, optionally, also taking into consideration the ohmic stator voltage drop caused by the stator current I and of the inductive stray voltage from the machine current, according to the equation EQU .psi.=.intg.(U-r.multidot.I)dt-x.multidot.I,
where r and x denote the machine parameters for the ohmic stator resistance and the stray inductance. Both the magnitude and the direction of flux are determined by treating .psi., U and I as vectors and evaluating the equation separately for the separate components of those vectors. To this end, for each component an integrating circuit is used to which the difference U-r I of the respective vector components is supplied. The product -x.multidot.I can then be imposed on the output of the integrator at a summing point, to take into consideration the inductive stray voltage as described in commonly-owned, U.S. Pat. No. 4,335,343.
With the aid of the flux parameter signal thus determined, the rotary field machine can be controlled in such a way that by presetting the stator current component parallel to the flux, the flux can be maintained at a given (i.e. a constant) desired value, while the torque can be controlled by monitoring the component perpendicular to the flux (so-called "field-oriented operation" of the machine).
A current measuring device connected to the machine terminal serves to supply the integrating circuit with a signal correlated with the ohmic stator resistance voltage drop r.multidot.I. During normal machine operation, the magnitude I of the stator current changes little, so that the level of this signal is always considerably greater than the magnitude of measurement error of the current sensing device. The device also provides for d-c isolation of the integrating circuit from the machine terminals. The machine voltage U is detected by a measurement transformer which under d-c isolation from the motor terminals has a transfer ratio adapted to the proportionality factor of the current sensor. The outputs of the current measuring device and voltage detector are processed by the integrator. The integrator performs the integration of u'-r.multidot.i' and optionally takes into account a quantity x.multidot.i' (also developed by the current sensor) to form a flux signal .psi. which is proportional to the respective components of the actual flux .psi.=.omega..sub.N .intg..multidot. (U-r.multidot.I)dt-x.multidot.I. When the flux .psi. is being maintained at a relatively high desired value the stator voltage must be correspondingly decreased for decreasing frequency. As the rotational speed approaches the lower limit of the intended normal operating frequency range, the load voltage level may drop to such an extent that a d-c component of magnitude u' (caused, for example, by the offset voltage and measurement errors of the instrument transformer) will lead to such inaccuracies in the input value u' and the calculated value that proper control of the machine will no longer be possible.